# Ridges in p-A (and pp) collisions

## TL;DR

This paper reviews the discovery of long-range particle correlations, called ridges, in small collision systems at the LHC, which resemble those in heavy-ion collisions and suggest possible collective behavior.

## Contribution

It summarizes experimental observations of ridge phenomena in small systems and discusses their implications for understanding collectivity and quark-gluon plasma formation.

## Key findings

- Ridges observed in pp and p-Pb collisions resemble those in Pb-Pb collisions.
- Ridge yield depends on transverse momentum, pseudorapidity, and multiplicity.
- Particle composition of ridges has been characterized.

## Abstract

Correlations between particles separated by several units of pseudorapidity were discovered in high-multiplicity pp and p-Pb collisions at the LHC. These long-range structures observed in two-particle correlation functions are reminiscent of features seen in Pb-Pb collisions, where they are often viewed as a signature of collective behavior and the formation of a quark-gluon plasma (QGP). Therefore, the discovery of these `ridges' in small systems has implications for the study of collectivity in small systems as well as in heavy-ion collisions. The ridges in pp and p-Pb collisions have been studied in the ALICE, ATLAS, CMS, and LHCb experiments to characterize the $p_{\rm{T}}$-, $\eta$-, and multiplicity-dependences of the ridge yield, as well as its particle composition.